Resistance of the organism to the stress injury changes during ontogenesis. In this regard, the incidence of cardiovascular, central nervous, endocrine systems, etc. increases at certain stages of individual development. Taking into account the fact that the development of stress-caused lesions is associated with accumulation of carbonyl products of free radical oxidation in cells, we have suggested that the one of the reasons for this phenomenon is age-related changes in the efficiency of the scavenging of endogenous aldehydes in the organism. In view of this fact, spectrum of aldo-keto reductases in blood serum of rats at different stages of ontogenesis was investigated by means of electrophoresis. Identical changes in the composition of aldo-keto reductases spectrum of blood in early immature age and in aging have been shown. Change in aldo-keto reductases spectrum modulates the role of reductive pathway of endogenous aldehydes scavenging in the organism. Thus, in child age and aging efficiency of scavenging of carbonyl products of free radical oxidation in cells is limited and especially when they are intensively produced. This results in increase of the susceptibility of the organism to oxidative stress.